Notes

After defeating Gwyn you can choose the ending you want. Lighting the bonfire will unlock the To Link the Fire ending. Exiting through the Fog Gate will unlock the Dark Lord ending. Note that you do not have to join Darkwraith to achieve the Dark Lord ending; only your choice at the end matters.

If you save Solaire by killing the red Chaos Bug in Lost Izalith shortcut (the one that needs +2 Chaos Servant covenant to unlock), you can summon him to help you. His summon sign is after the 3rd Black Knight (straight sword). Look left before you walk down the staircase. Solaire can help you kill the 2 remaining Black Knights.

You can fall off the stairs with the white tunnel effect, be careful if you plan to collect your bloodstain

Walk down the hill and follow the cliff edge, where you will encounter relatively thin concrete beams to walk along. At the pillar on the crossroads, be careful to hug the pillar as the physics can throw you off into the pit if you're not careful. Turning onto the right-side beam will lead to the Black Knight Set, or going straight on will lead you to the third Black Knight with another Black Knight Sword, who you will have to fight on the beams so be careful!

After passing along the beam that the third Knight guarded, follow the staircase down to face the Greataxe-wielding Black Knight (be careful of the broken stairs while avoiding his attacks)